1. Field of the Invention
The present invention relates to an apparatus for processing substrates such as semiconductor wafers with gas which has been made into plasma.
2. Description of the Related Art
The dry etching and ashing processes are carried out in the course of manufacturing semiconductor devices so as to etch and ash the surface of a substrate while using the reactivity of radicals in plasma. A wafer processing atmosphere is sometimes heated to increase the reactivity of radicals in the dry etching and ashing processes. In the case of the conventional ashing apparatus wherein wafers are individually processed, heaters are embedded in a wafer stage in the vacuum chamber to heat the wafer through the stage.
In the case where a plurality of wafers are to be dry-etched and ashed in the vertical process tube, however, the above-mentioned heater means of the conventional ashing apparatus wherein heaters are embedded in the wafer stage cannot be used because the flow of gas must be made uniform between the wafers.
Japanese Patent Disclosure Sho 62-196820 discloses an apparatus for removing foreign objects such as polysilicon, which become stuck to the inner wall of the process tube by the CVD process applied, from the process tube. According to this apparatus, electrical resistance heaters and a pair of RF electrodes are arranged along the outer circumference of the process tube. Temperature in the process tube is raised by the heaters, etching gas is introduced into the process tube, a high frequency electric field is added to the gas to generate plasma, and radicals are caused to act on the materials stuck to the inner wall of the process tube. As the result, the materials can be removed from a inner wall of the process tube. In the case where the heaters are arranged outside the process tube, however, it takes a long time to raise and lower the temperature in the process tube because the wafer processing atmosphere in the process tube is indirectly heated through the quartz-made wall of the process tube by the heaters.
In the case where an aluminium-made etch tunnel (or plasma shield member) is arranged along the outer circumference of the process tube, it becomes more difficult to quickly heat the wafer processing atmosphere because radiation heat is reflected by the etching tunnel. When the dry etching and ashing processes are to be carried out at high and normal temperatures in the same apparatus, therefore, it takes a long time for the temperature of the wafer processing atmosphere to be made stable. The throughput is thus lowered.
On the other hand, gas which is to be made into plasma is introduced into the apparatus through injectors. Each injector has a plurality of gas jetting holes arranged in the longitudinal direction of the processing tube. The electrical field, therefore, concentrates locally on each of the gas jetting holes to overheats the rim portion of it. The injector is usually made of quartz and there is cleaning into plasma. Therefore, the wafer processing atmosphere is contaminated by these quartz particles thus spread. In addition, the uniformity of process added to the wafers becomes low because the state of plasma is changed in the longitudinal direction of the injector.
Further, holes are formed all over the etching tunnel. A portion of the gas jetted through the injectors flows, as raw gas (or without being made into plasma), into the etching tunnel, passing through those holes of the etching tunnel which are opposite to the injectors and which are adjacent to them. The electric field in the etching tunnel is thus made thin and raw gas cannot be made into plasma in the etching tunnel. The amount of radicals is thus lowered in the wafer processing region. As a result, the etching rate is made so low that it is impossible to effectively process the wafers.